Pathogens that cause disease A summary of Chapter 7

Definitions Disease: Any change that impairs the normal function of an individual in some way. Parasite: An organism that lives on or in another organism and feeds from it, usually without killing it. Host: The organism or cell that a particular parasite lives in or on.

Definitions continued Pathogen: An organism that is able to cause disease in a host Vectors: An insect or other animal that carries a pathogenic organism from one host to another Infectious: Diseases caused by an agent that can be passed from one organism to another. Contagious: Diseases easily passed through the population.

Pathogens Types of pathogen can be split into TWO groups: Cellular pathogens are organisms that cause disease and include bacteria, protozoans, oomycetes, fungi, worms and arthropods. Non-cellular pathogens are not actually organisms but can cause disease and include viruses, viroids and prions.

Bacteria Bacteria are prokaryotic cells that are found almost everywhere. Some are autotrophs but most are heterotrophs. They can replicate very quickly by binary fission. Can be pathogens of both plants and animals. Some people can be infected by bacteria and not show symptoms, these people are known as carriers.

Bacteria classification

Staining allows us to differentiate between the two types of bacteria GRAM –ve bacteria stain pink. They have a complex multilayered cell wall that is made up of peptidoglycans as well as an additional outer membrane. These bacteria are generally less susceptible to penicillin. GRAM +ve bacteria stain purple. They contain a thick layer of peptidoglycans and are generally more susceptible to penicillin. NB: Peptidoglycans are a macromolecule only found in bacteria.

Bacteria can be transmitted by: Direct contact with an infected host Contact with objects/foods that have been contaminated Inhalation of dust or water droplets that have come into contact with, or produced by an infected host Contact with vectors carrying the bacterium.

How do bacteria cause disease? Some cause disease purely because their large numbers change normal functioning of their hosts tissue. Others directly attack and destroy host tissues. In other cases, the bacteria produce poisons (toxins) which may kill or seriously affect normal functioning of the hosts cells.

Common diseases caused by bacteria include: Anthrax Bubonic plague Salmonella poisoning Gonorrhoea Cholera Leprosy Syphilis

Controlling bacteria Bacteria can be controlled with a combination of: Antiseptics Disinfectants Antibiotics Antimicrobial agents.

Protozoa Protozoa are animal-like protists This means they are single-celled eukaryotic organisms The three classes of protozoa are classified according to their structure: a) flagellates. b) sporozoans. c) sarcodinians

The most common protozoa that cause disease Plasmodium – this is the organism responsible for malaria. It is classified as a sporozoan. It relies on a certain species of mosquito for its spread. It therefore has two hosts in its life cycle. The primary (adult) host is the human and the secondary (larval) host is the mosquito.

Plasmodium Life Cycle

Click on the image to see the animation

The most common protozoa that cause disease Trypnasoma – this protozoan is a flagellate and is the cause of African sleeping sickness. It relies on the tsetse fly for its transmission between hosts. Entamaoeba histolytica – this protozoan is a sarcodinian and is capable of causing a severe form of dysentery. It is spread through contamination of food or water. The disease called is known as amoebic dysentry.

Oomycetes These are fungus-like organisms that cause blight and downy mildew on plants. They were once thought to be fungi but they are motile and have many cellular processes not found in fungi so they are now classified in the kingdom Protista. When spores are released on a leaf they may be carried in water droplets to other leaves or they may be blown to reinfect the same host and to infect other plants.

Two main groups Yeasts: these are uni-cellular and can cause diseases such as thrush which is normally passed on by direct contact. Moulds: this category contains those fungi which consist of branching filaments known as hyphae. Typical members of this group which cause disease are rusts and smuts (particularly harmful to cereal crops) and human ringworm.

Fungi Fungi are heterotrophs and obtain their nutrients from decomposition of dead matter. Like bacteria, many species of fungi are important decomposers in biological systems. When fungi attack living plant or animal tissue, they can then be regarded as disease causing organisms.

Treatment Fungal infections can be treated with fungicides and antifungal treatments Many of these treatments have been found by accident.

Worms Parasitic worms can be classified as either: Flatworms (Platyhelmithes) such as tapeworms, hookworms and blood flukes. Roundworms (Nematodes) such as dog heartworm, Ascaris and filarial nematodes (which cause elephtiasis). In plants round worms infect roots and are major pests of orchards and crops.

Arthropods Insects are major vectors of disease in plants and animals. Ticks and mites are examples of arthropods that infect humans. Lerp insects are examples or arthropods that infect plants to produce galls.

Non-Cellular Pathogens These are infectious agents that cannot be regarded as organisms. These were poorly understood until the invention of the electron microscope in the late 1930’s.

Viruses Viruses infect all types of organisms. They are obligate intracellular parasites, meaning they cannot replicate outside of cells. A single virus particle (virion) is composed of genetic material either DNA or RNA, enclosed in a protein coat known as a capsid Viruses do not have cytoplasm, membranes or any organelles Viruses produce new virions by taking over the metabolic pathways of the host cell.

Classifying viruses Different features are used to classify viruses. These include: The kind of cell they use for reproduction The kind of nucleic acid they contain The different structures that make up the protein coat

Life cycle of a bacteriophage

Bacteriophage The type of virus that infects a bacterial cell. It’s protein coat becomes attached to the bacterial wall and its DNA is injected into the bacterial cell The phage DNA ‘takes over’ the bacterial cell and uses the energy and organelles of the cell to make new bacteriophages The host bacterial cell ruptures and the bacteriophages are available to infect other bacterial cells

Human viruses Details about the way in which mammalian cells are infected with viruses are not fully known. Some inject their nucleic acid in much the same way as bacteriophages. In other cases, the complete virus may be surrounded by a protein coat and enclosed by part of the plasma membrane of a cell.

Life cycle of a virus In some viral infections, the complete virus enters the infected cell. New viruses are assembled and then they leave the cell by budding from the cell within a portion of the hot cell’s membrane. This process does not necessarily kill the host cell and results in a persistent infection of cells as in the case of the herpes virus. http://www.hhmi.org/biointeractive/viral-lifecycle

Common Human Viruses Herpes complex HIV Chicken pox Influenza Measles Rabies Warts

Treatment Treating viral infections is more difficult that treating bacterial infections because it is difficult to attack a virus without attacking host cells as well. There has been some success with developing antiviral therapies that interfere with the un-coating of coated viruses, preventing nucleic acid synthesis and preventing the assembly and release of viral particles.

Prions Prions are infectious proteins that have only recently been recognised as being responsible for several degenerative brain diseases like mad-cow disease and the human disease Cruetzfeldt Jakob disease, which causes ‘holes’ to form in the brain tissue and leads to aggressive behaviour. Like viruses, prions are able to replicate in cells, but they are composed of protein only and contain no genetic material.

Viroids Viroids are tiny circular single-stranded RNA molecules, about one-tenth the size of the smallest virus. They have no protein coat or membrane capsule. They operate in a similar way to viruses. Viroids have so far only been identified in plants.

Have you heard about that awful mad cow’s disease? Yeah. It’s lucky I’m a horse! Have you heard about that awful mad cow’s disease?

How do prions infect? It appears that we all contain the genetic instructions to make normal prion protein. The protein occurs mainly in nerve cells and its function is unknown. If we become infected with a defective prion it converts normal protein into prion protein. This is the equivalent of a prion replicating itself. Cells do not ‘kill’ prions. Prions eventually cause a cell to burst and are free to infect other cells. The bursting of nerve cells results in the holes seen in infected brains.

Click on the image to the animation of how it is thought prions infect

Transmission Prions can be transferred by consumption of infected tissues, as occurred with the mad-cow epidemic in England were cow where fed a food supplement which contained sheep carcasses that were infected with scrapie. Human to human infection can also occur through body fluids There is a genetic link